Reference for Bekhorot 116:7

<big><strong>מתני׳</strong></big> באי זה צד מעשרין

But is not this obvious?<span class="x" onmousemove="('comment',' That he takes one from those born in Elul. For those born in Ab cannot help to tithe the others, in case Elul is a New Year and therefore there are only five of the previous year, a number insufficient for tithing. Again, he cannot take one lamb as a tenth from those born in Tishri for those born in Ab and Elul, in case Tishri is the New Year and therefore there are only five lambs, a number to which tithing cannot apply.');">⁷</span>

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